An obstacle is moving towards the source with velocity $v$. The sound is reflected from the obstacle. If $c$ is the speed of sound and $\lambda$ is the wavelength,then the wavelength of the reflected wave $(\lambda_{r})$ is

$A$ toy-car,blowing its horn,is moving with a steady speed of $5\, m/s$,away from a wall. An observer,towards whom the toy car is moving,is able to hear $5\, beats$ per second. If the velocity of sound in air is $340\, m/s$,the frequency of the horn of the toy car is close to ... $Hz$

An observer moves towards a stationary source of sound with a speed $\frac{1}{5}^{\text{th}}$ of the speed of sound. The wavelength and frequency of the waves emitted by the source are $\lambda$ and $f$ respectively. The apparent frequency and wavelength heard by the observer are respectively,

$A$ and $B$ are two sources generating sound waves. $A$ listener is situated at $C$. The frequency of the source at $A$ is $500 \, Hz$. $A$ now moves towards $C$ with a speed of $4 \, m/s$. The number of beats heard at $C$ is $6$. When $A$ moves away from $C$ with a speed of $4 \, m/s$,the number of beats heard at $C$ is $18$. The speed of sound is $340 \, m/s$. The frequency of the source at $B$ is ..... $Hz$.

Two vehicles,each moving with speed $u$ on the same horizontal straight road,are approaching each other. Wind blows along the road with velocity $w$. One of these vehicles blows a whistle of frequency $f_1$. An observer in the other vehicle hears the frequency of the whistle to be $f_2$. The speed of sound in still air is $V$. The correct statement$(s)$ is (are) :
$(A)$ If the wind blows from the observer to the source,$f_2 > f_1$.
$(B)$ If the wind blows from the source to the observer,$f_2 > f_1$.
$(C)$ If the wind blows from the observer to the source,$f_2 < f_1$.
$(D)$ If the wind blows from the source to the observer,$f_2 < f_1$.

$A$ train approaching a railway crossing at a speed of $120 \ km/h$ sounds a whistle of frequency $576 \ Hz$,when it is $288 \ m$ away from the crossing. The frequency heard by an observer standing on the road perpendicular to the track at a distance of $384 \ m$ from the crossing is (Speed of sound in air $= 340 \ m/s$): (in $Hz$)